Add virsh domif-setlink domif-getlink test

Introduce virsh domif-setlink domif-getlink  testing:

    1. Positive testing
        1.1 Running domain and shutting off domain testing
        1.2 Setlink domain interface up and down then check
        1.3 Options "--config" testing
        1.4 Interface name and MAC address testing

    2. Negative testing
        2.1 Invalid options testing
        2.2 Shutting off domain with interface name

Signed-off-by: whuang <whuang@redhat.com>
virt: add virsh functions for virsh domif-setlink and domif-getlink

Signed-off-by: whuang <whuang@redhat.com>
virt: add functions get  net devices of domain from guest xml

Signed-off-by: whuang <whuang@redhat.com>
modify some issue according to cevich's suggestion

Signed-off-by: whuang <whuang@redhat.com>